Heating system

ABSTRACT

A heating system for heating a fluid, the system comprising two separate heat exchangers, each directly heated by a burner to heat the fluid, a common flue into which the two burners are arranged to discharge their exhaust gas, a common fan arranged to supply air to the two burners, and a gas supply to each burner. A splitter valve splits the air flow from the fan to the two burners. A controller controls the speed of the fan, the position of the splitter valve and the gas supplies according to the heat output requirement of the system.

The present invention relates to a heating system.

Heating systems used for the domestic market (i.e., to provide central heating and hot water requirements) have heat exchangers which are mass produced. This leads to significant economies of scale. Larger heat exchangers for the adjacent market (small commercial and multi-occupancy residential buildings) tend to command a significantly higher price since, although this market is substantial, it is not as large and the same economies of scale do not arise.

Boilers for this adjacent market therefore tend to command significantly higher prices. One of the reasons for this is that it is not straightforward simply to “gang” several domestic boilers into a single flue and a single gas/air supply. The straightforward “adjacent mounting” of two boilers in parallel involves duplication of essentially all of the components of a single mounting.

It is possible to connect a pair of conventional burners to have a single common flue. If only one burner and heat exchanger is operating, the pressure of the exhaust gases in the common flue would tend to result in back flow through the other heat exchanger which is unacceptable. Consequently, a valve (for example, an on-off flapper valve) could be placed in the exhaust stream of each heat exchanger to prevent such back flow. The flapper valve corresponding to the operating burner is open whilst that of the non-operating burner is shut. Although such burners enable operation of a single burner, or both burners together, it is not practical to modulate either burner whilst both are running since there would be unequal back pressure and competition between the two fans.

According to the present invention, there is provided a heating system for heating a fluid, the system comprising:

two separate heat exchangers, each directly heated by a burner to heat the fluid;

a common flue into which the two burners are arranged to discharge their exhaust gas;

a common fan arranged to supply air to the two burners;

a gas supply to each burner;

a splitter valve to split the air flow from the fan to the two burners; and

a controller to control the speed of the fan, the position of the splitter valve and the gas supplies according to the heat output requirement of the system.

By using a splitter valve to split the flow, the gas flows into the burners are effectively isolated enabling the burners to operate more or less independently. There will, however, be some interaction between the two burners through the back pressure from the common flue. However, this can be compensated for by the controller. Effectively, the controller is programmed to compensate for the back flow for any given combination of fan speed and splitter valve position.

The splitter valve may be movable through a number of discrete positions to provide a number of discrete flow levels. However, preferably, the splitter valve is continuously variable over a range of positions. This provides greater control over the split of the streams.

Preferably, the splitter valve is of a type in which there is a substantially linear relationship between the position of a valve element and the ratio of fluid diverted to two streams. Such a valve is described in our earlier WO 2004/081362. This provides the advantage that a more robust flow control is provided across the range of movement of the valve.

The controller which controls the gas supply may do so directly, for example, using a dedicated valve in the gas line, or may do so indirectly, for example, using the speed of the fan to draw a controlled amount of gas into a venturi. In this second case, a valve would still be required in each gas supply to shut the gas supply off.

The valve may be a movable vane such as that described in WO 2004/081362. Alternatively, it may comprise an inlet; two outlets, one for each burner; an outer sleeve having two first outlet orifices, one for each stream; an inner element movably retained within the outer sleeve and having an inlet and two second outlet orifices, one for each stream; wherein the relative proportion of the inlet stream fed to each outlet is determined by the relative position of the inner element and outer sleeve. This arrangement is more robust than a rotatable vane as it does not work directly against the direction of flow in its extreme positions. Also, it is particularly suited to being able to provide the linear relationship between the valve position and flow as mentioned above. A valve of this type is disclosed in WO 2004/085893. An improved valve of this type including a laminarising plate, ramp surface, tapered outlet orifice, bleed hole and annular seal is disclosed in WO 2006/035238 and would also be suitable for use.

The gas supply may be introduced upstream of the splitter valve. Such an arrangement requires relatively few components, but does not allow independent control of the gas supply, other than by varying the position of the splitter valve. Therefore, preferably, gas is supplied to each burner downstream of the splitter valve. This allows a further degree of control of the gas supplied to each burner.

The principle may apply to more than two separate heat exchangers, each being heated by a burner. In this case, there may be a single splitter valve which is able to split the flow into more than two streams such as that disclosed in WO 2004/085893. Alternatively, there may be two or more valves connected in series, each valve capable of splitting the stream into two steams.

An example of a heating system in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of the heating system;

FIG. 2 is a perspective view of an outer sleeve of a splitter valve;

FIG. 3 is a similar perspective view of an inner sleeve of a splitter valve; and

FIG. 4 is a graphical representation of an output profile of the inner sleeve for a valve suitable for use in the system of FIG. 1.

The heating system comprises a first burner 1 and a second burner 2. Air to the burners is supplied from a single fan 3. This stream is split in a splitter valve 4 (described in greater detail below). Combustible gas is added to each of the air streams at gas venturi 5 under the control of respective valves 5A. Alternatively, it may be added via a single gas venturi upstream of the splitter valve 4.

Information about the demands of the burners 1, 2 is fed along control line 6 to the fan 3, splitter valve 4 and gas control valves 5. The speed of the fan 3, the position of the splitter valve 4 and the operation of the valves 5A are controlled accordingly, such that the requirements of the burners 1, 2 can be satisfied independently. The burners 1, 2 each provide heat to respective heat exchangers 7, 8 which may be arranged in series or in parallel. The heat exchangers carry a recipient fluid which receives heat from the burners 1, 2. The exhaust gases from the two burners 1, 2 are combined into a common flue 9.

The splitter valve will be described in greater detail with reference to FIGS. 2 and 3. FIG. 2 shows an outer sleeve 20 while FIG. 3 shows an inner sleeve 30 which, in use, is rotatably received within outer sleeve 20. Outer sleeve 20 has a screw threaded connection 21 which provides an inlet port in communication with the fan 3. Two similar screw threaded ports 22 and 23 corresponding to first 24 and second 25 outlets provide a connection for ducts leading to the two burners 1, 2. The two outlets 24, 25 are spaced axially along the sleeve and are both on the same side of the sleeve although they could be circumferentially offset. Each outlet has a first outlet orifice 26, 27 which is an axially extending elongate rectangular through aperture in the wall of the outer sleeve 20.

These first outlet apertures 26, 27 are shown in dashed lines in FIG. 3 for clarity.

In FIG. 3, the inner sleeve 30 is shown. The sleeve is hollow and has an inlet 31 at the end corresponding to the inlet port 21 to receive air from the fan 3. Second outlet orifices 32, 33 are elongate and are shown schematically to be generally triangular through orifices in the wall of the outer sleeve 30. The detail of the first and second outlet orifices 32, 33 is shown in FIG. 4. A gas seal (not shown) is provided in an annular groove 34 in the outer wall of the inner sleeve 30 between the second outlet orifices. This prevents flow from one outlet to the other between the outer 20 and inner 30 sleeves.

The inner sleeve 30 has a spindle 35 axially extending from the end opposite to the inlet 31. This is connected to a motor (not shown) allowing the inner sleeve 30 to be rotated about axis 36. Alternatively, rotation of the inner sleeve could be effected by a solenoid/electro-magnet contained within the outer sleeve 20. This latter option would enable the valve to be self-contained and therefore suitable for use with a fuel/air mixture which would allow the splitter valve 4 to be used downstream of the gas entry point, rather than upstream as shown in FIG. 1.

The operation of the valve will now be described with particular reference to the upper outlet 24. As the inner sleeve is rotated about axis 36 in the direction of arrow X, the second orifice 32 progressively overlaps to a greater and greater degree with the first orifice 26. It will be seen that there is a non-linear relationship between the rotary position of the inner sleeve 30 and the area of overlap such that during initial interaction between the first and second orifices, the area of overlap is relatively small (as compared to the case where second orifice has a similar rectangular shape to that of the first orifice).

The exact relationship is determined functionally to ensure that there is, as nearly as possible, a linear relationship between the rotational position of the inner sleeve 30 and the outlet flow.

A more detailed discussion of the relationship between the sizes of the orifices and the flow distribution of both streams is given in our earlier application WO 2004/081362.

It will be appreciated from FIG. 3 that as the sleeve 30 is rotated in the X direction, a greater proportion of flow is directed to the first outlet 24, while movement in the opposite Y direction causes more of the flow to be diverted to the second outlet 25.

The first and second orifices could be swapped, such that the rectangular orifice was provided on the inner sleeve and the non-rectangular orifice was provided on the outer sleeve. Alternatively, both orifices can be provided with a non-rectangular shape.

The precise control provided by the splitter valve 4 allows for a high level of modulation of each burner, and thus a high “turn-down” ratio of the complete appliance, without significant loss of efficiency of the system. These results can be achieved whilst using mass-produced heat-exchanger components so that the system can be produced cheaply. Further, since the burners 1, 2 can be arranged to alternately fire on successive occasions when only 1 burner is required, the life of the system can be enhanced.

The operation of the system will now be described with reference to FIG. 4.

Considered as a single appliance, the system, utilising two heat exchangers (each rated at 36 kW for the purposes of the illustration) would operate as follows:

1) Periods of high demand, (e.g. warming a large central heating system up from cold):

the fan 3 would be on a high setting, and the splitter valve 4 would have both outlet ports fully open. With the valve configuration shown in FIG. 4 the valve would be set to the 180° position. Both burners 1, 2 would run at maximum rating, and the overall appliance would provide the (maximum) thermal output—in this example 72 kW.

2) Periods of moderate, but constant, demand which is above the rating of a single heat exchanger, say 60% of maximum rating, or 43.2 kw.

a. The fan 3 would be set to 60% of the maximum flow, and the splitter valve 4 would be set to a 50:50 ratio (e.g. at 1800 orientation as shown in FIG. 4). It is noteworthy that in this operation mode, both burners are operating at 60% efficiency.

b. Alternatively, with the same fan setting, the splitter valve 4 could be positioned such that Output 1 is fully open, and Output 2 is approximately 20% open (this corresponds to an orientation of about 100° in FIG. 4). In this case, burner 1 is operating at its maximum rating (36 kW), and burner 2 at 20% of its rating (i.e., at 7.2 Kw).

-   -   The decision whether to operate under the first or second of         these conditions will depend on the efficiency characteristics         of the burners. It is well established that burners (with their         heat exchangers) are generally less efficient at lower operating         conditions. Whether it is more efficient to operate one burner         fully and the other at a significantly reduced level, or to         operate both at a partially reduced level, will depend on the         details of each burner. However, the system allows for full         control, and thus choice between the two.         3) Periods of moderate, but constant, demand which is only         marginally above the rating of a single heat exchanger, say 55%         of maximum rating. In this instance, it may not be possible to         modulate one burner down to 10% of its rated value; however, it         will be clear from the case above, that the system will enable         sharing of the demand, each running at 55%.         4) Periods of intermittent, medium demand (for instance,         restoring the temperature of domestic hot water supply during         summer). In this case, only one burner need be fired at any one         time. To “share” the loading on each burner/heat exchanger, the         burners are fired in sequence, swapping between the two each         time the demand returns. The valve (if configured as per FIG. 2)         thus will be alternately set to 85° and 265°.         5) Periods of intermittent high demand, with a background medium         level demand. By analogy with the previous case, the burners are         alternately switched off (thus the valve setting cycles between         the 90°, 270° and 180° positions). 

1. A heating system for heating a fluid, the system comprising: two separate heat exchangers, each directly heated by a burner to heat the fluid; a common flue into which the two burners are arranged to discharge their exhaust gas; a common fan arranged to supply air to the two burners; a gas supply to each burner; a splitter valve to split the air flow from the fan to the two burners; and a controller to control the speed of the fan, the position of the splitter valve and the gas supplies according to the heat output requirement of the system.
 2. A system according to claim 1, wherein the splitter valve is continuously variable over a range of positions.
 3. A system according to claim 2, wherein there is a substantially linear relationship between the position of a valve element of the splitter valve and the ratio of fluid diverted to two streams.
 4. A system according to claim 1, wherein the splitter valve comprises an inlet; two outlets, one for each burner; an outer sleeve having two first outlet orifices, one for each stream; an inner element movably retained within the outer sleeve and having an inlet and two second outlet orifices, one for each stream; wherein the relative proportion of the inlet stream fed to each outlet is determined by the relative position of the inner element and outer sleeve.
 5. A system according to claim 1, wherein gas is supplied to each burner downstream of the splitter valve.
 6. A system according to claim 1, comprising more than two separate heat exchangers, each being heated by a burner. 